1. Field of the Invention
The present invention relates to an array antenna system suited to be mounted on a moving platform such as a vehicle or a ship and provided with a function for tracking a target such as a satellite. Especially, the present invention relates to an array antenna system having a closed-loop antenna tracking system.
2. Description of the Prior Art
The INMARSAT (The International Maritime Satellite Organization) is known as the satellite communication system providing the world-wide mobile satellite communication services. Some other domestic mobile satellite communication systems have also been established.
In many of the mobile satellite communication systems (MSCSs), such as the INMARSAT, a subscriber can access the system using a mobile terminal with a directional antenna system. The directional antenna can be controlled to point to the satellite by an open-loop antenna control system or by a closed-loop tracking system. In the open-loop antenna control system, the direction (bearing angle) of the antenna can be controlled by using the information of the vehicle's direction and the feedback of the antenna bearing angle. For instance, the vehicle's direction is detected by a magnetic compass. However, it is noted that the information of the vehicle's direction from magnetic-compass is sometimes dismissed by the large errors on the vehicle.
On the other hand, the antenna can track the satellite by the receiving signal transmitted from a satellite by the closed-loop antenna tracking system. An example of the closed-loop antenna tracking system has been disclosed in "The 8-Element Spiral Array Antenna for the Mobile Satellite Communications" by Kuramoto et al., 1988 Spring National Meeting of IEICE, B-118. The disclosed antenna tracking system is referred to as the beam-switch antenna tracking (BST) system in azimuth.
In BST antenna system,
1) the antenna beam positions are frequently switched alternatively,
2) the tracking signal (the difference signal) is obtained by comparing with the received level signals (RECLEVs) at the two beam positions, and
3) the antenna is controlled to track the satellite by the tracking signal around the azimuth axis.
The BST antenna system is suitable for tracking the target which transmits the carrier signal continuously under the condition of no blockages in line-of-sight. However, it is not possible to track the satellite by the BST under the conditions of no receiving signals from the target by blockages in line-of-sight.
The number of MSCSs, such as the INMARBAT, which adopt the burst-mode SCPC/TDM carrier system will increase. In general, the TDM (Time Division Multlplexed) carrier is a continuously transmitted carrier broadcasting the channel information to the subscribers for the access and control, and the burst-mode SCPC (Single Channel Per Carrier) carrier is a burst-mode (voice-activated) transmission carrier for voice signal transmission from base-station to subscriber via satellite.
It is also not suitable for the primitive BST to track the satellite by the tracking signal obtained from the burst-mode SCPC carrier.
To solve the problems of closed-loop antenna tracking systems during the intervals of no receiving signal, an hybrid tracking method has been proposed (refer to the Japanese Patent Laid-Open No. Hei 5-232206). In this patent application document, it is suggested that the antenna bearing angle is controlled by using the output of an angular rate-sensor during short intervals of no receiving signal. Though it is not clearly explicit how to use the output of the angular rate-sensor for controlling the antenna bearing angle in the above document, it is supposed that vehicle's turning angle may be estimated at real-time by integrating the output of an angular rate-sensor fixed to the vehicle and the antenna may be controlled to compensate the above estimated turning angle by using the feedback signal of the antenna bearing angle from the potentiometer/encoder of the azimuth axis during no-receiving signal intervals. Therefore, the proposed hybrid tracking method will offer better performances than primitive closed-loop method will under no receiving signal condition. However, the proposed hybrid tracking method seems to have some problems to be overcome. The first problem expected will be caused by the offset voltage of angular rate-sensor output (It is not always easy to cancel the offset voltage because of its drift.). The vehicle's turning angle may be estimated by integrating the output of an angular rate-sensor fixed to the vehicle. However, the large angle error may be generated by the integration of the offset voltage (and its drift) of angular rate-sensor output. It may be possible to avoid a large error due to an offset voltage by adopting a low-drift angular rate-sensor. However, it will be costly to use a low-drift rate-sensor. The second problem expected is that it will be costly to obtain the feedback signal of the antenna bearing angle from the potentiometer/encoder of the azimuth (AZ) axis. The 3rd problem expected is that a time lag (delay) for the control-mode transition (from closed-loop control to open-loop control, or vice versa) is required because the decision of the mode-transition depends on the receiving signal condition and/or a timer and it (time lag) may degrade the tracking performances during mode-transitions.